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# Metrics Streaming
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The primary servlet for streaming Hystrix command and thread pool metrics in text/event-stream format for real-time monitoring and dashboard integration.
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## Capabilities
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### HystrixMetricsStreamServlet
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Main servlet that provides SSE streaming of Hystrix dashboard metrics including circuit breaker states, latency percentiles, and thread pool statistics.
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```java { .api }
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/**
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 * Streams Hystrix metrics in text/event-stream format.
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 * Install by including hystrix-metrics-event-stream-*.jar in classpath
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 * and configuring in web.xml with URL pattern /hystrix.stream
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 */
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public class HystrixMetricsStreamServlet extends HystrixSampleSseServlet {
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    /**
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     * Default constructor using HystrixDashboardStream and default delay
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     */
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    public HystrixMetricsStreamServlet();
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    /**
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     * Package-private constructor for testing with custom stream and delay
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     * @param sampleStream Observable stream of dashboard data
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     * @param pausePollerThreadDelayInMs Delay between polling cycles in milliseconds
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     */
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    HystrixMetricsStreamServlet(Observable<HystrixDashboardStream.DashboardData> sampleStream, int pausePollerThreadDelayInMs);
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    /**
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     * Returns maximum number of concurrent connections allowed (configurable via hystrix.config.stream.maxConcurrentConnections)
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     * @return Maximum concurrent connections (default: 5)
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     */
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    protected int getMaxNumberConcurrentConnectionsAllowed();
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    /**
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     * Returns current number of active connections
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     * @return Current connection count
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     */
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    protected int getNumberCurrentConnections();
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    /**
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     * Atomically increments and returns current concurrent connection count
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     * @return New connection count after increment
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     */
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    protected int incrementAndGetCurrentConcurrentConnections();
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    /**
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     * Atomically decrements current concurrent connection count
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     */
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    protected void decrementCurrentConcurrentConnections();
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}
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```
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**Web.xml Configuration:**
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```xml
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<servlet>
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  <description></description>
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  <display-name>HystrixMetricsStreamServlet</display-name>
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  <servlet-name>HystrixMetricsStreamServlet</servlet-name>
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  <servlet-class>com.netflix.hystrix.contrib.metrics.eventstream.HystrixMetricsStreamServlet</servlet-class>
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</servlet>
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<servlet-mapping>
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  <servlet-name>HystrixMetricsStreamServlet</servlet-name>
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  <url-pattern>/hystrix.stream</url-pattern>
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</servlet-mapping>
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```
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**Usage Examples:**
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```java
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// Basic servlet deployment - just configure in web.xml
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// No additional Java code required
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// Testing servlet with curl
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curl http://localhost:8080/app/hystrix.stream
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// Custom configuration via system properties
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System.setProperty("hystrix.config.stream.maxConcurrentConnections", "10");
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```
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## Data Format
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The servlet outputs JSON events in Server-Sent Events format:
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### HystrixCommand Metrics
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```json
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data: {
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  "type": "HystrixCommand",
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  "name": "PlaylistGet",
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  "group": "PlaylistGet", 
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  "currentTime": 1355239617628,
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  "isCircuitBreakerOpen": false,
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  "errorPercentage": 0,
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  "errorCount": 0,
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  "requestCount": 121,
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  "rollingCountSuccess": 121,
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  "rollingCountFailure": 0,
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  "rollingCountTimeout": 0,
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  "rollingCountShortCircuited": 0,
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  "rollingCountThreadPoolRejected": 0,
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  "rollingCountSemaphoreRejected": 0,
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  "currentConcurrentExecutionCount": 0,
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  "latencyExecute_mean": 13,
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  "latencyExecute": {
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    "0": 3, "25": 6, "50": 8, "75": 14, "90": 26, "95": 37, "99": 75, "99.5": 92, "100": 252
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  },
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  "latencyTotal_mean": 15,
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  "latencyTotal": {
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    "0": 3, "25": 7, "50": 10, "75": 18, "90": 32, "95": 43, "99": 88, "99.5": 160, "100": 253
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  },
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  "propertyValue_circuitBreakerRequestVolumeThreshold": 20,
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  "propertyValue_circuitBreakerSleepWindowInMilliseconds": 5000,
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  "propertyValue_circuitBreakerErrorThresholdPercentage": 50,
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  "propertyValue_executionIsolationStrategy": "THREAD",
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  "propertyValue_executionIsolationThreadTimeoutInMilliseconds": 800,
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  "reportingHosts": 1,
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  "threadPool": "PlaylistGet"
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}
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```
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### HystrixThreadPool Metrics
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```json
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data: {
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  "type": "HystrixThreadPool",
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  "name": "ABClient",
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  "currentTime": 1355239617628,
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  "currentActiveCount": 0,
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  "currentCompletedTaskCount": 4459519,
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  "currentCorePoolSize": 30,
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  "currentLargestPoolSize": 30,
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  "currentMaximumPoolSize": 30,
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  "currentPoolSize": 30,
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  "currentQueueSize": 0,
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  "currentTaskCount": 4459519,
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  "rollingMaxActiveThreads": 13,
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  "rollingCountThreadsExecuted": 919,
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  "propertyValue_queueSizeRejectionThreshold": 30,
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  "propertyValue_metricsRollingStatisticalWindowInMilliseconds": 30000,
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  "reportingHosts": 3
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}
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```
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## Configuration Properties
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```java { .api }
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// System properties for configuration
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public static final String MAX_CONCURRENT_CONNECTIONS_PROPERTY = "hystrix.config.stream.maxConcurrentConnections";
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public static final int DEFAULT_MAX_CONCURRENT_CONNECTIONS = 5;
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```
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## Error Handling
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- Returns HTTP 503 when maximum concurrent connections exceeded
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- Graceful handling of client disconnections via PrintWriter.checkError()
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- Automatic resource cleanup when servlet is destroyed
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- Thread-safe connection counting using AtomicInteger